or, indeed, through anything where it is necessary to make sure that no accident can happen, or where great precision of position and form of hole is required, I find a boring tube mounted as shown in the picture (Fig. 36) is of great service. Brass or iron tube borers do perfectly well, and the end of the spindle may be provided once for all with a small tube chuck, or the tubes may be separately mounted as shown. A fairly high speed is desirable, and may be obtained either by foot, or, if power is available, is readily got by connecting to the speed cone of a lathe, which is presumably permanently belted to the motor.
Fig. 36.
After trying tubes armed with diamond dust, as will be presently explained, I find that emery and thin oil or turpentine, if liberally supplied below the glass, will do very nearly as well. The tube should be allowed to rise from the work every few seconds, so as to allow of fresh emery and oil being carried into the circular grooves. This is done by lifting the hinged upper bearing, the drill being lifted by a spiral spring between the pulley and the lower bearing shown at B. The glass may be conveniently supported on a few sheets of paper if flat, or held firm in position by wooden clamps if of any other shape. In any case it should be firmly held down and should be well supported. Any desired pressure upon the drill is obtained by weighting the hinged board A.
§ 40. The following method was shown to me by Mr. Wimshurst, but I have not had occasion to employ it myself. It is suitable for boring large holes through such glass as the plates of Mr. Wimshurst's Influence machines are usually made of. A diamond is mounted as the "pencil" of a compass, and with this a circle is drawn on the glass in the desired position. The other leg of the compass of course rests on a suitable washer.
To the best of my recollection the further procedure was as follows. A piece of steel rod about one-eighth inch in diameter was ground off flat and mounted in a vice vertically, so as to cause its plane end to form a small horizontal anvil. The centre (approximately) of the diamond-cut circle of the glass was laid on this anvil so as to rest evenly upon it, and the upper surface (i.e. that containing the cut) was then struck smartly with a hammer, completely pulverising the glass above the anvil. The hole was gradually extended in a similar manner right up to the diamond cut, from which, of course, the glass broke away.
A similar method has long been known to glaziers, differing from the preceding in that a series of diamond cuts are run across the circle parallel to two mutually perpendicular diameters. A smart tap on the back of the scored disc will generally cause the fragments to tumble out. I have never tried this myself, but I have seen it done.
Large discs may easily be cut from sheet glass by drawing a circular diamond cut, and gradually breaking away the outer parts by the aid of additional cuts and a pair of pliers or "shanks" (see Fig. 44).
[§ 41. Operations depending on Grinding: Ground-in Joints. —]
The process will be perfectly understood by reference to a simple case. Suppose it is desired to grind the end of a tube into the neck of a bottle. If a stoppered bottle is available, the stopper must be taken out and measured as to its diameter at the top and bottoM. Select a bit of tube as nearly as possible of the same diameter as the stopper at its thickest part. Draw down the glass in the blow-pipe flame rather by allowing it to sink than by pulling it out. After a few trials no difficulty will be experienced in making its taper nearly equal to that of the stopper, though there will in all probability be several ridges and inequalities. When this stage is reached anneal the work carefully and see that the glass is not too thin. Afterwards use emery and water, and grind the stopper into the bottle.